The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, MR imaging in inhomogeneous magnetic fields.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The use of MR in musculoskeletal (MSK) diagnostics is a rapidly growing field. Arthroplasty is the surgical placement of implants. The population of patients having some form of metal implant is quite large and growing rapidly. MR has significant capabilities in assisting the diagnosis of implant revisions. Using magnetic resonance imaging to assist in clinical diagnostics of MR-compatible arthroplastic implants, however, has proven a fundamentally challenging problem. Most materials that are robust and durable enough to utilize for bone replacements will have magnetic properties that, when placed in a typical B0 magnetic field, induce extraneous fields of amplitude and spatial variation that are large compared to the field offsets utilized in conventional spatial encoding. Accordingly, these materials can introduce distortions in the main magnetic field resulting in an inhomogeneous magnetic field.
While the signal loss induced by these field gradients can largely be regained through the use of Hahn spin-echoes, the distortion they produce in both the readout and slice directions are drastic and are typically unacceptable for clinical evaluation. Despite these challenges, MRI has been shown to be quite useful in the diagnosis of degenerative conditions in arthroscopic patients. In particular, MRI has been used to screen perioprosthetic soft tissues, diagnose osteolysis, and visualize implant interfaces. These diagnostic mechanisms benefit significantly from visual information near implant interfaces. Unfortunately, artifacts induced by the implants in conventional MRI images are most severe near the implant interfaces.
A proposed approach to reducing MRI artifacts induced by implants is 2D FSE imaging using View-Angle Tilting (VAT). Though this approach can improve in-plane distortions at the cost of significant image blurring, it does not address distortions in the slice-selection direction. Near the most paramagnetic of utilized metallic implants, distortions in the slice-selection direction can almost completely disfigure 2D MR images. While a slice-distortion correction of VAT images in the slice direction has been proposed, its use is limited because it does not correct signal-pileup effects of image distortion.
It would therefore be desirable to have a system and method capable of reducing image artifacts near or around implant interfaces. It would further be desirable to improve clinical diagnostic access to regions of interest near or around implant interfaces.